You can only cast spells on regions you own or are adjacent to.

Each spell has a minimum cost, this is the minimum amount of fury required to cast any particular spell.

If 2 spells are cast on the same region by opposing players, the spell with the highest amount of fury spent will take effect, the losing player will lose any fury spent.
If 2 spells are cast on the same region for the same amount, neither spell will take effect & both players will lose any fury spent.

If spells are cast on adjacent regions, the spell with the lower amount of fury spent is processed 1st, followed by the higher fury spell, so, if two spells are cast on neighboring regions and they have conflicting effects, the last spell processed will override the effects of any previous spells.

There are seven groups of spells in Evernight. The following chart lists the name and uses for each group.

The Black between: Used to upgrade Lessers and to end the game of Evernight.

Ancient Amra: Used to alter defensive combat dynamics.

Church of the Transcendent Body: Used to change region types.

Druids of Oarne: Used for various misc effects.

Generation Technology: Used to effect movement.

Gonmol Horde: Used to alter offensive battle dynamics.

Psylicides: Used to manipulate visual information.

Check out this link for a more detailed description
http://wyrmmlore.com/evernight/spellintro.html